A Phase Locked Loop (“PLL”) circuit is an electronic control system that may be used, in part or in whole, to generate or maintain one signal that is locked onto the phase and frequency of another signal. While particular embodiments of a PLL may vary with respect to their component parts and devices, an example PLL may include devices and components such as a frequency/phase detector, a charge pump, a loop filter, and a variable-frequency signal generator (e.g., a voltage controlled oscillator (“VCO”)). In an example situation, a PLL may receive a input data signal from an outside source (e.g., an input signal from a remote data source) and may compare the phase of the input data signal with a locally-generated variable-frequency clock signal. The locally-generated variable-frequency clock signal may originate, for example, from a variable-frequency signal generator over which the PLL has control. As part of the comparison, a phase detector within the PLL may determine a phase difference between the input data signal and the variable-frequency clock signal and may use the phase difference to generate a phase correction signal that will ultimately increase or decrease the phase of the VCO.
For example, after the phase detector has detected a phase difference between the input data signal and the clock signal and has generated a corresponding phase correction signal, the phase correction signal may be input into a charge pump which generates a current in response to the phase correction signal. In turn, the current generated by the charge pump may be filtered by a loop filter and may dictate the output voltage of the loop filter (e.g., a large current from the charge pump translates to a large output voltage of the loop filter). In particular embodiments, the output voltage of the loop filter controls the frequency of the clock signal generated by the VCO. Thus, by controlling the phase of the VCO using phase correction signals, the PLL may adjust the phase of the VCO to match the phase of the input data signal.
One of ordinary skill in the art will appreciate that the above-described example was presented for the sake of explanatory simplicity and that PLLs which use feedback to maintain the frequency/phase of a VCO-produced signal in specific relationship with an input data signal may be used in frequency synthesizers and in a host of other applications.